I googled "The value of a thing is just as much as it will bring." in quotes, and got several hits that show Marx referred to Samuel Butler, the author of Hudibras.
Here is one.
Here is a another, see Inscription.
I'm not sure what the "fuss" is. Is TRPF really so heavily disputed? In a sense, profits are just a kind of arbitrage, and so the reasoning behind TRPF is similar to the reasoning around zero-arbitrage assumptions (profit-maxing agents under perfect competition with free entry/exit and full information will drive supply up and prices down until the profit ...